The invention relates to an electrode system, particularly for electrochemical sensors, which comprises a working electrode, a counterelectrode and an electrolyte.
Such an electrode system is used for example in the oxygen electrode of Clark.
The oxygen electrode of Clark is used for measuring the partial pressure of oxygen in the blood, among other uses. The blood gas analysis is an important branch of medical diagnostics and is able to give evidence of the state of the cardiovascular system as well as of metabolic processes in the organism.
For determining the partial pressure of oxygen in the blood and other biological media there are used sensor systems based either on the principle of electrochemical sensor technology, such as amperometry, or on the principle of dynamic fluorescence quenching. The oxygen electrode of Clark is a representative of the first-named group.
The principle of the oxygen electrode of Clark is based on the reaction (reduction) of O2 to OHxe2x88x92 on a platinum electrode in the presence of H2O. As anodic counterreaction, the release of silver ions on a silver anode is usually, but not exclusively, utilized.
The service life of such a sensor system depends on a series of factors. However, a limitation that is essential in practice arises due to the deposition at the cathode of the silver released at the anode. Thereby, the polarizability of the cathode is suppressed, which leads to unwanted side-reactions and an increasing inactivation of the electrode. As a rule, an electrode thus inactivated can only be regenerated by mechanical measures. However, while miniaturizing an oxygen electrode of Clark and making it maintenance-free as desired, a mechanical regeneration becomes nearly impossible.
A number of possibilities are known to extend the service life of a sensor system of the kind of the oxygen electrode of Clark. On the one hand, it is tried to keep the silver concentration as low as possible by means of appropriate salt solutions. On the other hand, the diffusion of the silver to the cathode is inhibited as well as possible by mechanical and chemical measures. These, however, are only dilatory measures.
Another possibility of extending the service life consists in the use of gold or platinum as anode material. However, such anodes are polarizable and, as a consequence, do not give stable potentials in the long run. The service life of such electrode systems is several weeks. If, on the contrary, non-polarizable metals are used, there again occur corresponding depositions at the cathode and/or surface changes at the anode.
The silver deposition problem is particularly serious in miniaturized oxygen electrodes that are manufactured according to the principle of the planar technique, where the immediate vicinity of the silver anode to the platinum cathode as well as the small amount of electrolyte that is available result within a very short time in the inactivation of the cathode, due to silver deposition.
In EP-A-0 603 154 for example there is described an amperometric enzyme electrode for measuring the concentration of an enzyme substrate, wherein the decomposition of H2O2 to H+ and oxygen is measured amperometrically in a known manner. The electrode material of the working electrode consists in a redox-inactive conductor with a conductive pigment, a binding agent that is not conducting itself and a catalytically active substance finely distributed therein. As catalytically active substance, manganese dioxide deposited on graphite or activated charcoal may be contemplated, for example. The catalytically active substance suppresses interferences when the enzyme substrate is measured. As counterelectrode, a conventional silver/silver chloride electrode with the known disadvantages is used in this known system.
The invention has as its object to avoid the problems of the known electrode systems and to provide an electrode system which has an improved long-term stability. In particular, it should be possible to use the electrode system for a miniaturized oxygen electrode.
According to the invention, in an electrode system, particularly for electrochemical sensors, which comprises a working electrode, a counterelectrode and an electrolyte, this object is achieved in that the counterelectrode is constituted by a material containing an elementary carbon.
In the electrode system according to the invention, the service life has shown itself as being considerably increased, which is due to the fact that at the working electrode there do not occur depositions, which diminish the polarizability of the working electrode and lead to unwanted side-reactions at the working electrode. At the same time, however, the potential of the counterelectrode remains stable over a long time period thanks to the inventive measure.
Although an electrode constituted by a material containing a carbon exhibits, in principle, the phenomenon of polarizability like an electrode with gold or platinum as electrode material, it has surprisingly been found that the long-term stability of the electrode system according to the invention is clearly higher than that of a conventional electrode system.
According to a preferred embodiment of the invention, the counterelectrode is connected as anode.